Delignifying lignocellulose with an incomplete soda cook followed by gaseous bleaching

ABSTRACT

A process is provided for the preparation of pulp from lignocellulosic material. It involves two essential steps, namely: firstly, carrying out a modified conventional sodacooking step in which the cooking conditions are deliberately selected to be such that the yield is significantly higher than that normally corresponding to a bleachable pulp and in which the material is simultaneously at least partially defiberized; and secondly, either partially or completely delignifying the pulp by subjecting the pulp so produced to a gaseous bleaching treatment while it is in defiberized form.

United States Patent Inventors David W. Clayton Hudson; Raimbault M. A.T. De Montigny, Baie DUrfe; Norman Liebergott, Laval, all of CanadaAppl. No. 767,829 Filed Oct. 15, 1968 Patented Nov. 2, 1971 AssigneePulp and Paper Research Institute of Canada Pointe Claire, Quebec,Canada Priority Dec. 15, 1967 Canada 007,696

DELIGNIFYING LIGNOCELLULOSE WITH AN INCOMPLETE SODA COOK FOLLOWED BYGASEOUS BLEACHlNG 18 Claims, No Drawings (1.8. CI 162/19, 162/24,162/25, 162/63, 162/66, 162/67, 162/89 Int. Cl D21c 3/02, D210 3/18Field of Search 162/89, 19,

I361 Tieferences Cited UNITED STATES PATENTS 1,890,179 12/1932 l-lelder162/25 2,186,034 1/1940 Murdock 162/89 3,472,731 10/1969 Liebergott eta1. 162/89 OTHER REFERENCES Casey, .l. P Pulp and Paper, 2nd Edition,Volume I, p. 102, lnterscience Pub. 1960.

Primary Examiner-Samih N. Zahama Assistant Examiner-Thomas G. ScavoneAltomeyCraig, Antonelli, Stewart & Hill .pulp by subjecting the pulp soproduced to a gaseous bleaching treatment while it is in defiberizedform.

DELIGNIFYING LIGNOCELLULOSE WITH AN INCOMPLETE SODA COOK FOLLOWED BYGASEOUS BLEACHING This invention relates to the production of pulp. Thepulp may be fonned from any suitable lignified vegetable matter,examples of which include softwood, hardwood, bagasse, cereal straws andbamboo. In particular, this invention is directed to the production ofpartially delignified pulp, partially bleached pulp and fully bleachedpulp. When used in the present specification, the term partiallydelignified pulp" is intended to mean the product obtained by thepartial removal of the residual lignin remaining in the pulp afterdigestion, by means other than by digestion.

Softwood soda pulping to produce easily bleachable pulp has been foundto be no longer practical since to produce softwood soda pulps ofacceptably low lignin content, a very long cooking period is required inwhich the pulps obtained suffer severe degradation and a high yieldshrinkage. The strength properties of such pulp are substantiallyinferior to those of kraft pulp. Soda pulps of higher lignin content arevery resistant to bleaching by conventional liquid phase bleachingtechniques, and, in fact, it has been found that the quantity ofchemicals required to achieve bleaching is far too great to becommercially attractive.

Consequently, it has been common practice to produce pulps of highstrength by means of kraft pulping. It has been the practice to cookkraft pulps to yields of 43-48 percent, the unbleached pulps therebyhaving maximum physical strength properties. Within this yield range,the pulps are bleachable. It is then possible to bleach the kraft pulpsin the above-mentioned 43-48 percent yield range, to obtain strengthproperties of the bleached pulps which are not greatly different fromthose of the unbleached pulps.

However, kraft pulping suffers the principal disadvantage of theproduction of malodors which occur due to the formation of hydrogensulfide and organic sulfur compounds during the kraft cook. Thenecessity for reducing or eliminating such air pollution from kraftmills has now become a serious and costly problem.

A principal object, therefore, of the present invention is to provide aprocess for producing partially dilignified pulps, partially bleachedpulps, and fully bleached pulps using soda pulping, such pulpsbeingsuitable as a replacement for unbleached kraft pulps, semibleached kraftpulps and fully bleached kraft pulps.

An object of another aspect of this invention is to provide a processfor the production of pulp wherein air pollution by malodors is nolonger a factor.

By a broad aspect of this invention, a process is provided for thepreparation of pulp which comprises. the steps of: (a) cooking alignified vegetable material in an alkaline solution containingpredominantly sodium hydroxide in a modified cooking step at atemperature, and for a time sufficient to give a yield significantlyhigher than that normally corresponding to a bleachable pulp for thelignified vegetable material being treated; and (b) at least partiallydelignifying the cooked pulp in defiberized form by means of ahigh-density gaseous treating procedure.

lt has been found that the combination of the two steps set forth abovegave the advantage that the strength properties of the treated pulpswere considerably higher than those of the unbleached pulps. Anunexpected advantage of the combination of the two steps set forth aboveis that Elrepho brightness of about 90 percent can be accomplished in asfew as the following five high-density stages, namely: gaseouschlorination; gaseous ammonia extraction; gaseous chlorine dioxideoxidation; gaseous ammonia extraction; and gaseous chlorine dioxideoxidation. Also, the shrinkage in high-density treatment is less than inconventional bleaching.

It has been found that the cooking in the alkaline liquor must be for atime, and at a temperature, sufficient to give a yield significantlyhigher than that normally corresponding to a bleachable pulp for thematerial being treated. This yield thus can vary depending on thematerial being treated. Thus,

for a softwood, the yield significantly higher than that normallycorresponding to a bleachable pulp is at least 48 percent, to give apulp having a lignin content more than 5 percent. The minimum yields forother lignified vegetable material are given in the following table:

Minimum Percent The first essential step of the process of broad aspectsof this invention is a modified conventional soda-cooking step. In otherwords, the step is carried out under conditions well known to thoseskilled in the art, with the modification that the cooking conditionsare deliberately selected to be such that the yield is significantlyhigher than that normally corresponding to a bleachable pulp for thelignified vegetable material being treated. These modified conditionscan be readily chosen by a person skilled in the art. Exemplarymodifications of the process steps include a reduced cooking time,reduced liquor concentration, reduced maximum cooking temperature, orany combinations thereof.

The second essential step of the process of broad aspects of thisinvention is that the pulp produced by the modified sodacooking step issubjected to a high-density gaseous treatment, while it is indefiberized form, in order to delignify the pulp, either partially orcompletely. After the modified cooking step, then, all or a portion ofthe cooked material may be subjected to a defiberization treatment. Thecooked material may be screened and if the quantity of rejects is lessthan about 2 percent, the rejects may be discarded or be returned to thecooking step, and the accepted material may be directly subjected to theaforementioned high-density gaseous treatment. If the rejects are higherthan about 2 percent, they would be defiberized and added to theaccepted material. This defiberization treatment is most usuallyeffected by mechanical means, such as by the action ofa disc refiner.

As mentioned hereinabove, an essential step in the process ofthisinvention is that the cooked pulp, in defiberized form, is thendelignified, either partially or completely by means of a high-densitygaseous treatment. One such high-density gaseous treatment may becarried out according to a first aspect of this invention by treatingthe pulp with gaseous chlorine followed by an alkaline extractionaccording to the following procedure. Such process for the high-densitygaseous treatment comprises pretreating the cooked pulp by the steps of:(l) continuously passing the cooked pulp which should be in the form ofmoist fluff through a first zone of an atmosphere containing excesschlorine; (2) limiting the residence time within that first zone(usually for a time of the order of 20 seconds to 5 minutes) to such anextent that less than the potential amount of chlorine is acquired bythe flufi (and usually until about 50-85 percent of the potential amountof chlorine is acquired by the fluff); and (3) continuously passing thefluff including the chlorine which has been acquired by the fluffthrough a second zone of an essentially chlorine-free atmosp'here undersuch retention time conditions as to permit the amount of said chlorineto react with said fluff (usually for a time of about 1 to 15 minutes);and then carrying out the ad ditional steps of: (4) washing thechlorinated pulp; and (5) further treating the washed pulp with analkaline reagent.

A preferred alkaline reagent is gaseous ammonia. in which case thealkaline treatment step is preferably carried out as follows: l) washingthe pretreated pulp and finely dividing tion time generally of the orderof about 1 to about 30 minutes;.and (4) maintaining the temperatureduring the reaction time within the range, usually, of about 60"-100 C.

Another high-density gaseous delignification treatment according to asecond aspect of this invention is by treating the soda pulp withgaseous chlorine dioxide followed by an alkaline extraction according tothe following procedure. Such process for the high-density gaseoustreatment comprises pretreating the cooked pulp by the steps of: (1)treating the defiberized pulp which should be in the form of a fluff,generally at a consistency of 20-60 percent with chlorine dioxide, whichmay contain varying proportions of chlorine gas, diluted to less than100 mm. mercury partial pressure with air, nitrogen, steam or an inertgas, at a retention time generally of the order of 20 seconds to 60minutes and at a temperature which usually varies between 15 C. and 100C.; and then carrying out the additional steps of (2) washing thepretreated pulp; and (3)further treating the washed pulp with analkaline reagent.

A preferred alkaline reagent is gaseous ammonia, in which case thealkaline treatment step is preferably carried out as follows: (1)washing the pretreated pulp and finely dividing the washed pulp intofiber aggregates generally at a moisture content of about 40-85 percent;(2) preheating the fiber aggregates to a temperature generally in therange of 60-l00 C.; (3) exposing the preheated fiber aggregates to anatmosphere comprising gaseous ammonia, either undiluted or diluted withsteam, air, nitrogen, or other inert gas for a reaction time generallyof the order of about 1 to about 30 minutes; and 4) maintaining thetemperature during the reaction time within the range, usually, of about60-100 C.

By another aspect of this invention, the pulp after being partiallydelignified may be subjected to further delignification,

and bleaching, in further high-density stages, with chlorine dioxide,according to the following procedure. Such process for the furtherhigh-density gaseous treatment comprises the steps of: (1) adjusting themoisture content of the fibers to a level usually within the range offrom 60 to 70 percent; (2) exposing the fiber aggregate to chlorinedioxide gas diluted with an inert solvent where the partial pressure ofchlorine dioxide is 100 mm. of mercury or less, for a period whichusually ranges from about 5 to about 30 minutes at a temperature whichusually ranges from about 65 to about 100 C.; and 3) controlling thepH-of the mixture so that final pH is preferably in the range offrom 3.5to 6.5.

The following is a description of an experiment designed to produce twopulps by means of a modified soda pulping cycle which is one of theessential steps of the process of a broad aspect of this invention.

EXPERIMENT A 42 g./l.). The liquor-to-wood ratio was 5:1. The digesterwas brought up to maximum temperature, 170 C., within 90 minutes. Onecook (Pulp No. l) was terminated after 30 minutes of cooking(temperature 170C. and 100 p.s.i.g. pressure); the other (Pulp No. 11)after 10 minutes of cooking (temperature 170 C. and 100 p.s.i.g.pressure). The resultant pulps were fiberized by action of a discrefiner(one pass ata plate separation of 0.01 inch). Cooking and defiberizingproduced pulps with the following characteristics:

Roe Elrepho "Visual Yield. Kappa chlorine brightness, efficleney,

Pulp No percent No. N 0. percent percent new GTE 561a Ram's Pulp I PulpII (52. 4% yield) (63.3% yield) Beating revolutions (PFI mill) 12, 80017, 600 32, 000 39, 500 Canadian standard freeness (cc.) 500 300 500 300Bulk (co/gm.) 1. 40 1. 38 1.68 l. 52 Breaking length (m 10, 800 12, 2508, 400 9, 000 Burst factor 85. 0 92. 5 64. 5 70. 5 Tear factor 115 107120 108 1, 880 2, 080 700 840 Stretch (percent) 3. 68 3. 63 3. 33 3. 70

The following examples set forth the treatment of the pulps produced bya modified soda cooking technique of one step of this inventionaccording to several different high-density gaseous treating techniquesembodying several aspects of the process of this invention.

EXAMPLE 1 Strength Properties Beating Revolutions 7,000 13,200 CanadianStandard Freeness 500 300 (cc.) Bulk (cm/g.) 1.33 L28 Breaking Length(M) 14,050 14,050 Burst Factor 117.5 123.0 Tear Factor 1 18 I14 MIT Fold2,930 3,010 Stretch (11) 4.08 4.0!

EXAMPLE 11 One portion of the chlorinated, extracted and washed pulpproduced in example 1 was pressed to 35 percent consistency. The pressedpulp was sprayed with a solution of sodium carbonate equal to 1.25percent ofthe weight of pulp (B.D. basis) and sufficient water to reducethe consistency to 30 percent. The pulp was then shredded and heated toC. The

. shredded and heated pulp was then treated with chlorine dioxide gasdiluted to 30 mm. pressure by addition of nitrogen; 1 percent chlorinedioxide on the dry weight of fiber was added to the pulp. The pulp wasmaintained at the reaction temperature of 80 C. for 30 minutes, thenremoved and washed. Characteristics of the semibleached pulp were:brightness, 76.4 percent; visual efficiency, 87.7; yield on wood, 47.3percent.

Strength Properties One portion of the semibleached pulp obtained inexample 11 was shredded at 20 percent consistency and treated with 0.5percent of its weight of ammonia (in a mixture of ammonia and steam) for1 minute at C. and washed. The pulp was then shredded at 30 percentconsistency and preheated to' 80 C., then treated with chlorine dioxidegas diluted to 30 mm.

pressure by the addition of nitrogen; 0.4 percent chlorine dioxide onthe weight of dry fiber was added to the pulp; retention time was 30minutes. Characteristics of the pulp after washing were: brightness 91.1percent; visual efficiency 94.3 percent; yield on wood 46.9 percent.

Strength Properties Beating Revolutions EXAMPLE 1V Another portion ofPulp No. l was shredded. at 30 percent consistency and treated withgaseous chlorine dioxide diluted with nitrogen; 4.3 percent chlorinedioxide on the weight of dry fiber was added to the pulp, maximumtemperature was 60 C., retention time 1 minute. After washing, the pulpwas shredded at percent consistency and treated with 2.5 percent ofammonia based on dry fiber (in a mixture of ammonia and steam) for 1minute at 100 C. and washed again. Characteristics of the partiallydelignified pulp were: Kappa number 4.2, Elrepho brightness 45, visual,efiiciency 58.9 percent, yield on wood 48.6 percent.

EXAMPLE V One portion ofPulp No. 11 was shredded at 30 percentconsistency and treated with gaseous chlorine (158% C1 on pulp o.d.basis), maximum temperature C. for 1 minute,

washed, shredded at 20 percent consistency and treated with 4.5 percentof ammonia based on dry fiber (in a mixture of steam ammonia) for 1minute at C. and washed again.

Characteristics of the partially delignified pulp were: Kappa number14.7, brightness 36, visual efficiency 53.7 on the scale of the Elrephometer, and yield on wood 48.6 percent.

Strength Properties Beating Revolutions 5,400 11,200 Canadian StandardFreeness 500 300 Bulk (cc./g.) 1.31 1.27 Breaking Length (M) 13,50013,900 Burst Factor 1 I2 Tear Factor 92 90 MIT Fold 2,240 2.740 Stretch3.70 3.95

EXAMPLE Vl Another portion of Pulp No. 11 was shredded at 30 percentconsistency and treated with gaseous chlorine dioxide diluted withnitrogen; 6.5 percent chlorine dioxide on the weight of dry fiber wasadded to the pulp, maximum temperature was 97 C., retention time 1minute. After washing, the pulp was shredded at 20 percent consistencyand treated with 4.5 percent of ammonia based on dry fiber (in a mixtureof ammonia and steam) for 1 minute at 100 C. and washed again.Characteristics of the partially delignified pulp were: Kappa number4.8, Elrepho brightness 44, visual efiiciency 57.2 percent, yield onwood 52.7 percent.

Strength Properties A comparison of the physical properties at 300 CSFof an unbleached kraft pulp with the same physical properties of twopartially delignified sodapulpsis shown below. It should be noted thatthe physical properties of the delignified soda pulps are similar tothose of the unbleached kraft pulp, yet the 0 yield of the sodavpulpsare 2 to 3 percent higher.

The comparisons are set forth in table 11:

TABLE II Total Breaking yield, Bulk, length Burst Tear Stretch, MIT PulpNo. percent cc./gm. (m.) factor factor percent fold Kraft pulp R 45.3 1. 23 14, 800 122. 0 111 4.0 2,900 Soda pulp:

A 1 47. 4 1. 28 14, 050 123. 0 11-1 -1. 08 3, 010 1-B 1 c 48. 6 1. 2415, 800 131. 0 101 4. 00 2, 900

We c1515 1. A sulfur-free process for the preparation of pulp whichcomprises the steps of:

A. cooking ands ubstantially simultaneously at least par,- tiallydefiberizing a lignified vegetable material in an alkaline solutioncontaining predominantly sodium hydroxide in a modified cooking step, ata temperature, and for a time sufficient to give a yield significantlyhigher than that normally corresponding to a bleachable pulp for thelignified vegetable material being treated, said significantly higheryield being correlated to the nature of the lignified 7 vegetablematerial according to the following relation ship:

Vegetable Minimum Lignin Minimum Yield Material Content (5) (I) Sofiwood5 48 Hardwood 2-4 50 Ba'gasse 6 50 Straws 6 50 Bamboo 5 45 B.'at leastpartially delignifying the cooked pulp when it is in 5. The process ofclaim 4 wherein the softwood is mechanically defiberized.

6. The process of claim 2 wherein step (B) comprises pretreating thepulp by l. continuously passing the defiberized pulp in the form ofmoist fluff which is at a moisture content of about 4085 percent througha first zone of an atmosphere containing excess chlorine;

2. limiting the residence time within that first zone to such an extentthat less than the potential amount of chlorine is acquired by thefluff;

. continuously passing thefluff including the chlorine which has beenacquired by the fluff through a second zone of an essentiallychlorine-free atmosphere under such retention time conditions as topermit the amount of said chlorine to react with said fluff;

and effecting the further steps of:

4. washing the pretreated pulp; and

5. further treating the washed pulp with an alkaline reagent.

7. The process of claim 6 wherein the treatment with the alkalinereagent comprises:

i. finely dividing the pretreated pulp into fiber aggregates at amoisture content of about 40-85 percent;

ii. preheating the fiber aggregates to a temperature of about 60- 1 OObLC.;

iii. exposing the preheated fiber aggregates to an atmosphere comprisinggaseous ammonia for a reaction time of about 1 to about 30 minutes; and

iv. maintaining the temperature during the reaction time within therange of 60- 1 C.

8. The process of claim 6 wherein the residence time in the first zoneis 20 seconds to minutes.

9. The process of claim 6 wherein the fluff acquires from 50-85 percentof the potential amount of chlorine.

10. The process of claim 6 wherein the residence time in the second zoneis l-l5 minutes.

11. The process of claim 8 wherein the fluff acquires from 5085 percentof the potential amount of chlorine and further wherein the residencetime in the second zone is l-l5 minutes.

12. The process of claim 2 wherein step (C) comprises the step ofpretreating the defiberized pulp in the form of a flufi which is at amoisture content of -40 percent with chlorine dioxide diluted to lessthan mm. mercury partial pressure with a nonreactive gas, at a retentiontime of the order of 20 seconds to 60 minutes and at a temperature whichvaries between 15 C. and 100 C.; and effecting the further steps of (2)washing the pretreated pulp and (3) further treating the washed pulpwith an alkaline reagent.

13. The process of claim 12 wherein the treatment with the alkalinereagent comprises:

i. finely dividing the pretreated pulp into fiber aggregates at amoisture content of about 40-85 percent;

ii. preheating the fiber aggregates to a temperature of about 60-l00bLC.;

iii. exposing the preheated fiber aggregates to an atmosphere comprisinggaseous ammonia for a reaction time of about 1 to about 30 minutes; and

iv. maintaining the temperature during the reaction time within therange of 60-l00 C.

14. The process of claim 12 wherein the nonreactive gas is air,nitrogen, steam, or an inert gas.

15. The process of claim 7 wherein the ammonia is diluted with steam,air, nitrogen, or other inert gas.

16. The process of claim 12 wherein the ammonia is diluted with steam,air, nitrogen, or other inert gas.

17. The process of claim 7 including the further steps of:

l. adjusting the moisture content of the fibers to a level of from 60 to70 percent;

[1. exposing the fiber aggregate to chlorine dioxide gas diluted with aninert solvent where the partial pressure of chlorine dioxide is 100 mm.of mercury or less, for a period of about 5 to about 30 minutes at atemperature from about 65 to about 100 C.; and

Ill. controlling the pH of the mixture so that final pH is in the rangeoffrom 3.5 to 6.5.

18. The process of claim 12 including the further steps of:

l. adjusting the moisture content of the fibers to a level of from 60 to70 percent;

ll. exposing the fiber aggregate to chlorine dioxide gas diluted with aninert solvent where the partial pressure of chlorine dioxide is 100 mm.of mercury or less, for a period of about 5 to about 30 minutes at atemperature from about 65 to about 100 C.; and Ill. controlling the pHof the mixture so that final pH is in the range from 3.5 to 6.5.

* I i i i

2. limiting the residence time within that first zone to such an extentthat less than the potential amount of chlorine is acquired by thefluff;
 2. The process of claim 1 including the positive additional stepof defiberizing the cooked lignified vegetable matter after it has beensubjected to said modified cooking step but before it has been subjectedto step (B).
 3. The process of claim 2 wherein the alkaline solution issoda liquor.
 3. continuously passing the fluff including the chlorinewhich has been acquired by the fluff through a second zone of anessentially chlorine-free atmosphere under such retention timeconditions as to permit the amount of said chlorine to react with saidfluff; and effecting the further steps of:
 4. washing the pretreatedpulp; and
 4. The process of claim 2 wherein the lignified vegetablematerial is softwood and the yield is at least 48 percent.
 5. Theprocess of claim 4 wherein the softwood is mechanically defiberized. 5.further treating the washed pulp with an alkaline reagent.
 6. Theprocess of claim 2 wherein step (B) comprises pretreating the pulp by 7.The process of claim 6 wherein the treatment with the alkaline reagentcomprises: i. finely dividing the pretreated pulp into fiber aggregatesat a moisture content of about 40-85 percent; ii. preheating the fiberaggregates to a temperature of about 60*-100* C.; iii. exposing thepreheated fiber aggregates to an atmosphere comprising gaseous ammoniafor a reaction time of about 1 to about 30 minutes; and iv. maintainingthe temperature during the reaction time within the range of 60*-100* C.8. The process of claim 6 wherein the residence time in the first zoneis 20 seconds to 5 minutes.
 9. The process of claim 6 wherein the fluffacquires from 50-85 percent of the potential amount of chlorine.
 10. Theprocess of claim 6 wherein the residence time in the second zone is 1-15minutes.
 11. The process of claim 8 wherein the fluff acquires from50-85 percent of the potential amount of chlorine and further whereinthe residence time in the second zone is 1-15 minutes.
 12. The processof claim 2 wherein step (C) comprises the step of pretreating thedefiberized pulp in the form of a fluff which is at a moisture contentof 80-40 percent with chlorine dioxide diluted to less than 100 mm.mercury partial pressure with a nonreactive gas, at a retention time ofthe order of 20 seconds to 60 minutes and at a temperature which variesbetween 15* C. and 100* C.; and effecting the further steps of (2)washing the pretreated pulp and (3) further treating the washed pulpwith an alkaline reagent.
 13. The process of claim 12 wherein thetreatment with the alkaline reagent comprises: i. finely dividing thepretreated pulp into fiber aggregates at a moisture content of about40-85 percent; ii. preheating the fiber aggregates to a temperature ofabout 60*-100* C.; iii. exposing the preheated fiber aggregates to anatmosphere comprising gaseous ammonia for a reaction time of about 1 toabout 30 minutes; and iv. maintaining the temperature during thereaction time within the range of 60*-100* C.
 14. The process of claim12 wherein the nonreactive gas is air, nitrogen, steam, or an inert gas.15. The process of claim 7 wherein the ammonia is diluted with steam,air, nitrogen, or other inert gas.
 16. The process of claim 12 whereinthe ammonia is diluted with steam, air, nitrogen, or other inert gas.17. The process of claim 7 including the further steps of: I. adjustingthe moisture content of the fibers to a level of from 60 to 70 percent;II. exposing the fiber aggregate to chlorine dioxide gas diluted with aninert solvent where the partial pressure of chlorine dioxide is 100 mm.of mercury or less, for a period of about 5 to about 30 minutes at atemperature from about 65 to about 100* C.; and III. controlling the pHof the mixture so that final pH is in the range of from 3.5 to 6.5. 18.The process of claim 12 including the fUrther steps of: I. adjusting themoisture content of the fibers to a level of from 60 to 70 percent; II.exposing the fiber aggregate to chlorine dioxide gas diluted with aninert solvent where the partial pressure of chlorine dioxide is 100 mm.of mercury or less, for a period of about 5 to about 30 minutes at atemperature from about 65 to about 100* C.; and III. controlling the pHof the mixture so that final pH is in the range from 3.5 to 6.5.